Imports System
Imports System.Collections
Public Class SamplesArrayList
Public Shared Sub Main()
Dim myStr As [String]
' Creates and initializes a new ArrayList.
Dim myAL As New ArrayList()
myAL.Add("red")
myAL.Add("orange")
myAL.Add("yellow")
' Creates a read-only copy of the ArrayList.
Dim myReadOnlyAL As ArrayList = ArrayList.ReadOnly(myAL)
' Displays whether the ArrayList is read-only or writable.
If myAL.IsReadOnly Then
Console.WriteLine("myAL is read-only.")
Else
Console.WriteLine("myAL is writable.")
End If
If myReadOnlyAL.IsReadOnly Then
Console.WriteLine("myReadOnlyAL is read-only.")
Else
Console.WriteLine("myReadOnlyAL is writable.")
End If
' Displays the contents of both collections.
Console.WriteLine()
Console.WriteLine("Initially,")
Console.WriteLine("The original ArrayList myAL contains:")
For Each myStr In myAL
Console.WriteLine(" {0}", myStr)
Next myStr
Console.WriteLine("The read-only ArrayList myReadOnlyAL contains:")
For Each myStr In myReadOnlyAL
Console.WriteLine(" {0}", myStr)
Next myStr
' Adding an element to a read-only ArrayList throws an exception.
Console.WriteLine()
Console.WriteLine("Trying to add a new element to the read-only ArrayList:")
Try
myReadOnlyAL.Add("green")
Catch myException As Exception
Console.WriteLine(("Exception: " + myException.ToString()))
End Try
' Adding an element to the original ArrayList affects the read-only ArrayList.
myAL.Add("blue")
' Displays the contents of both collections again.
Console.WriteLine()
Console.WriteLine("After adding a new element to the original ArrayList,")
Console.WriteLine("The original ArrayList myAL contains:")
For Each myStr In myAL
Console.WriteLine(" {0}", myStr)
Next myStr
Console.WriteLine("The read-only ArrayList myReadOnlyAL contains:")
For Each myStr In myReadOnlyAL
Console.WriteLine(" {0}", myStr)
Next myStr
End Sub 'Main
End Class 'SamplesArrayList
'This code produces the following output.
'
'myAL is writable.
'myReadOnlyAL is read-only.
'
'Initially,
'The original ArrayList myAL contains:
' red
' orange
' yellow
'The read-only ArrayList myReadOnlyAL contains:
' red
' orange
' yellow
'
'Trying to add a new element to the read-only ArrayList:
'Exception: System.NotSupportedException: Collection is read-only.
' at System.Collections.ReadOnlyArrayList.Add(Object obj)
' at SamplesArrayList.Main()
'
'After adding a new element to the original ArrayList,
'The original ArrayList myAL contains:
' red
' orange
' yellow
' blue
'The read-only ArrayList myReadOnlyAL contains:
' red
' orange
' yellow
' blue

Remarks

A collection that is read-only does not allow the addition, removal, or modification of elements after the collection is created.

A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection; therefore, if changes are made to the underlying collection, the read-only collection reflects those changes.